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DigitQ

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description

DigitQ["string"]

Gives True if every character of "string" is a digit 0-9, and False otherwise. DigitQ[""] is True.

Examples (13)

Every input below was run against the current Mathilda build and its output recorded.

Basic examples (4)

In[1]:= LetterQ["αβγ"]
Out[1]= True

In[2]:= DigitQ["\:0662"]
Out[2]= False

In[3]:= UpperCaseQ["ABC"]
Out[3]= True

In[4]:= LowerCaseQ["abc1"]
Out[4]= False

Applications (9)

In[5]:= DigitQ["0123456789"]
Out[5]= True

In[6]:= DigitQ["12a"]
Out[6]= False

A sign is not a digit

In[7]:= DigitQ["-12"]
Out[7]= False

Nor is a decimal point

In[8]:= DigitQ["1.5"]
Out[8]= False

Arabic-Indic digits: DigitQ is ASCII-only

In[9]:= DigitQ["١٢٣"]
Out[9]= False

While the pattern DigitCharacter accepts them

In[10]:= StringMatchQ["١٢٣", DigitCharacter..]
Out[10]= True

Vacuously True

In[11]:= DigitQ[""]
Out[11]= True

In[12]:= Select[{"2024", "v2", "007", ""}, DigitQ]
Out[12]= {"2024", "007", ""}

A number is not a string: DigitQ::string

In[13]:= DigitQ[123]
Out[13]= DigitQ[123]

Implementation notes

Algorithm. builtin_digitq calls charq_builtin(res, "DigitQ", CQ_DIGIT), which requires one String argument (otherwise DigitQ::string via strings_arg_message, unevaluated), then decodes the UTF-8 code points one at a time with u8_decode and returns False at the first one for which char_ok(CQ_DIGIT, cp) fails — so the scan short-circuits — and True if none does (vacuously for ""). char_ok answers ASCII code points inline (0–9) and returns 0 for every non-ASCII code point without consulting any table — DigitQ is ASCII-only, as in Mathematica. Inside a string pattern, predicate_class in string_pattern.c translates _?DigitQ to the PCRE class [[:digit:]] instead of emitting a per-character callout.

Data structures. None beyond the decoder: the test is a pure range check on the code point.

Complexity / limits. O(L) for an ASCII string; any non-ASCII character fails immediately. Note the asymmetry with the string pattern DigitCharacter, which (compiled with PCRE2 UCP) matches every Unicode decimal digit. Characters are code points, not grapheme clusters. Not Listable; no packed-array or Compile[] surface (the argument is a String).

Attributes: Protected.

References

See also: LetterQ, UpperCaseQ, LowerCaseQ

Notes & additional examples

Notes

  • True exactly when every character is an ASCII digit 0–9, as in Mathematica — deliberately narrower than the string pattern DigitCharacter, which matches any Unicode decimal digit.
  • True for "". A non-string emits DigitQ::string and stays unevaluated; not Listable.
  • DigitQ is a test on text, not a number test: use IntegerQ / NumberQ for expressions.
  • See also: DigitCharacter, LetterQ, ToExpression.